Reproduction signal evaluation method

ABSTRACT

A reproduction signal evaluation method of this invention includes the step of obtaining a reproduction signal from an information recording medium on which digital information is recorded with record marks having different sizes, the step of obtaining the amplitude of a first reproduction signal, of the signals contained in the reproduction signal, which reflects digital information recorded with a record mark having the maximum size, the step of obtaining the amplitude of a second reproduction signal, of the signals contained in the reproduction signal, which reflects digital information recorded with a record mark having the second smallest size, the step of obtaining an evaluation value from the ratio of the amplitudes of the first and second reproduction signals, and the step of evaluating a characteristic of the reproduction signal on the basis of the evaluation value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. application Ser. No. 10,899,140, filedJul. 27, 2004, which is a divisional of U.S. application Ser. No.09/826,907, filed Apr. 6, 2001, and is based upon and claims the benefitof priority under 35 U.S.C. §119 from Japanese Patent Application No.2000-106639, filed Apr. 7, 2000; the entire contents of both of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a reproduction signal evaluation methodof evaluating the quality of a reproduction signal reproduced from aninformation recording medium on which digital information is recorded.The present invention also relates to an information recording medium onwhich digital information is recorded. In addition, the presentinvention relates to an information reproduction apparatus forreproducing digital information from an information recording medium onwhich digital information is recorded.

Recent years have seen a remarkable increase in the recording density ofinformation recording media such as optical disks and magnetic disks.With this increase in recording density, the quality margin ofreproduction signals read out from media by a digitalrecording/reproduction apparatus is reduced. For this reason, the stateof each record mark on a medium must be accurately specified. In thecase of exchangeable media, in particular, consideration must be givenin advance to the facts that a single medium is used in a plurality ofapparatuses, and a plurality of media are used in a single apparatus.Compatibility must be ensured between the media. That is, thecharacteristics of a reproduction signal obtained when a medium isplayed in a predetermined apparatus must be specified. In other words,reproduction signals must be accurately evaluated; otherwise,specifications for compatibility cannot be provided for media.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation and, has as its object to provide the following reproductionsignal evaluation method, information recording medium, and informationreproduction apparatus: (1) a reproduction signal evaluation methodcapable of accurately evaluating reproduction signals obtained by aninformation recording medium on which predetermined information isrecorded at a high density; (2) an information recording medium on whichpredetermined information is recorded at a high density under apredetermined condition such that reproduction signals satisfying apredetermined evaluation condition can be obtained; and (3) aninformation reproduction apparatus for reproducing information from aninformation recording medium on which predetermined information isrecorded at a high density under a predetermined condition such thatreproduction signals satisfying a predetermined evaluation condition canbe obtained.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a view showing the schematic arrangement of the reproductionsignal processing system of an optical disk apparatus using PRML;

FIG. 2 is a chart showing a reproduction signal waveform and PRequalization;

FIG. 3 is a view showing a PR equalization circuit using a transversalfilter;

FIG. 4 is a chart showing identifying operation based on the PRMLscheme;

FIG. 5 is a graph showing an example of the MTF characteristic of anoptical disk apparatus;

FIG. 6 is a graph showing a reproduction signal waveform in ahigh-density optical disk apparatus;

FIG. 7 is a graph showing the frequency distribution of sample values ofa reproduction signal after PR equalization;

FIG. 8 is a flow chart showing an outline of a reproduction signalevaluation method using a signal before waveform equalization;

FIG. 9 is a flow chart showing an outline of a reproduction signalevaluation method using a signal after waveform equalization;

FIG. 10 is a view showing the schematic arrangement of the reproductionsignal processing system of an optical disk apparatus using a levelslice scheme;

FIG. 11 is a chart showing a reproduction signal waveform andidentifying operation using a level slice;

FIG. 12 is a graph showing a reproduction signal evaluation based onjitter; and

FIG. 13 is a graph showing a reproduction signal waveform in the opticaldisk apparatus shown in FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

An optical disk apparatus using a level slice scheme will be describedfirst with reference to FIG. 10.

Digital data is recorded on an optical disk 100. Tracks are formed bydigital data strings recorded on the optical disk 100. The optical disk100 includes a recording/reproduction disk on which data can be written,a reproduction-only disk formed by recording data in recess/projectionform on a disk surface in a manufacturing process, and a composite diskhaving both a recordable area and a reproduction-only area.

In the reproduction mode, the optical disk 100 is rotated by a motor101, and an optical head 102 irradiates the rotated optical disk 100with a laser beam. More specifically, the optical head 102 has an LD(laser diode), and the light beam emitted from the LD is focused on arecord mark on a track formed on the optical disk 100 via an objectivelens. Reflected light from the optical disk 100 is focused on aphotodetector via a condenser lens to be converted into a reproductionsignal. Note that the above record mark indicates both a mark recordedon a phase change recording layer with a change in reflectance and anembossed pit in recess/projection form.

The signal (based on the reflected light) output from the optical head102 is amplified by a reproduction amplifier 103 and iswaveform-equalized by a waveform equalizing section 104. The waveformequalizing section 104 is formed by a filter having a high frequencyemphasis characteristic and the like to facilitate identification ofdigital data continuously recorded along a track.

To reconstruct digital data recorded on an optical disk into theoriginal data, the reproduction signal is converted into binary data of0 or 1, and the data contained in the reproduction signal is identifiedby establishing synchronization on the basis of a clock signal. No clocksignal is recorded on the optical disk. For this reason, a PLL circuit105 generates a clock signal from the reproduction signal. Meanwhile, awaveform slice identification circuit 106 outputs binary data insynchronism with the clock signal output from the PLL circuit 105. As anidentification method, a waveform slice scheme of discriminating 0 from1 with reference to the central level of the reproduction signal byusing a level comparator.

The waveform slice scheme will be described with reference to FIG. 11.For example, record marks (“(c)” in FIG. 11) are formed on an opticaldisk in accordance with a recorded data waveform (“(b)” in FIG. 11)corresponding to recorded data (“(a)” in FIG. 11). When information(record mark) recorded on the optical disk is to be reproduced, the LDemits a reproduction light beam as a small beam spot (the round hatchedportion in “(c)” in FIG. 11), and reflected light of this light beam isdetected, thereby reading the record mark. That is, a reproductionsignal is obtained. The waveform (reproduction waveform) of thisreproduction signal is obtained as a rounded waveform like the oneindicated by “(d)” in FIG. 11 instead of a rectangular waveform like therecorded data waveform indicated by “(b)” in FIG. 11.

The equalizer therefore performs waveform equalization such that eachintersection of an equalization waveform and a given threshold(indicated by the chain line) is located in the center of anidentification point, as indicated by “(e)” in FIG. 11. Morespecifically, the high-frequency component of the reproduction signal isamplified. With regard to the comparison result on the equalizationwaveform and the threshold at each identification point, anidentification unit sets “1” if the level of the equalization waveformis higher, and “0” otherwise, thereby output binary data. As aconsequence, decoded data is obtained, as indicated by “(f)” in FIG. 11.

As evaluation criteria for optical disks and drives, jitter afterwaveform equalization, a modulation amplitude before waveformequalization, and asymmetry are used.

A method of defining jitter will be described below. An intersection ofan equalization waveform and a threshold does not necessarily coincidewith the center of a window due to noise and the like. For this reason,an intersection between the equalization waveform and the threshold isdistributed with respect to an identification point interval (window),as shown in FIG. 12. In this case, jitter is expressed by the standarddeviation of the intersection data normalized with the window width.

A modulation amplitude and asymmetry will be described next. Owing tothe resolution characteristic (MTF), the reproduction signal based on asmall record mark with a high density on an optical disk has a smallamplitude, whereas the reproduction signal based on a large mark with alow density on the disk has a large amplitude. If the size of a recordmark becomes equal to or larger than a size corresponding to a spot sizeon the optical disk, the amplitude of a reproduction signal becomesalmost constant. As a consequence, reproduction signals from an opticaldisk on which record marks having different sizes are formed becomethose shown in FIG. 13.

In this case, a modulation amplitude (MA) is defined by the ratio of theamplitude of a signal with the highest density (densest signalamplitude: 10) to the amplitude of a signal with the lowest signalamplitude (coarsest signal amplitude: 11). The densest signal amplitudeis the amplitude of a signal with the highest density. This signal hasthe shortest signal level change period and the smallest signalamplitude. The coarsest signal amplitude is the amplitude of a signalwith the lowest density. This signal has the longest signal level changeperiod and a large signal amplitude. Since signals with low densitieslower than a certain density are almost equal in amplitude, such anamplitude can be considered as the maximum value of the amplitudes ofall reproduction signals. A modulation amplitude can be expressed by theratio of these two values as follows:

Asymmetry (SA) is defined by the offset amount between the central levelof a densest signal and the central level of the coarsest signal.Asymmetry can be given from the low-level side (L0L) and high-level side(L0H) of a densest signal and the low-level side (L1L) and high-levelside (L1H) of a coarsest signal amplitude according to:

These definition schemes properly function when an enough amplitude isensured for a densest signal, and at the same time, a sufficiently highS/N ratio is ensured in terms of quality. In practice, even if data wasidentified according to a binary waveform slice, the amplitude of adensest signal could be measured as long as the error rate wassufficiently low.

As the recording density increases, however, the amplitude of a densestsignal decreases, and the signal becomes difficult to identify by abinary waveform slice. In addition, adjacent data interfere witch eachother to make it difficult to separate the data. These phenomena willincrease the error rate, resulting in an inability to read stored data.

As a means of coping with such phenomena accompanying an increase inrecording density, the PRML (Partial Response and Maximum Likelihood)scheme is available. PRML is a data identification means replacing thebinary waveform slice scheme. The PRML scheme can eliminate theinfluence of interference between data, and even if the quality of adensest signal deteriorates to a certain degree, reproduce morelikelihood data from the preceding and succeeding data.

Even if such a new means of coping with an increase in recording densityis used, a decrease in the densest signal amplitude of a reproductionsignal and a deterioration in quality are inevitable. Under thecircumstances, accurate evaluation of a reproduction signal is difficultto perform on the basis of the definitions of a modulation amplitude andasymmetry depending on a densest signal. In addition, according to theevaluation method based on a binary waveform slice, an evaluation resultcannot be properly correlated with an error rate after identification inspite of the change in identification method. This makes it impossibleto correctly reflect a request in a reproduction signal from a medium.

As described above, according to the definition methods for a modulationamplitude, asymmetry, and the like in association with reproductionsignals, it is difficult to accurately evaluate reproduction signals.

If the PRML scheme is used to cope with an increase in recordingdensity, it is impossible to properly correlate an evaluation resultwith an error rate after identification on the basis of the definitionsbased on a binary waveform slice.

If a reproduction signal cannot be accurately evaluated, it isimpossible to specify record marks on a recording medium to ensurecompatibility. Therefore, an evaluation method exhibiting a properrelationship with an error rate after identification must be specified.

FIG. 1 is a view showing reproduction signal processing in an opticaldisk apparatus using the PRML scheme corresponding to a high-densityoptical disk.

Digital data are recorded on the optical disk 100. Tracks are formed bythe digital data strings recorded on the optical disk 100. The opticaldisk 100 includes a recording/reproduction disk on which data can bewritten, a reproduction-only disk formed by recording data inrecess/projection form on a disk surface in a manufacturing process, anda composite disk having both a recordable area and a reproduction-onlyarea.

In the reproduction mode, the optical disk 100 is rotated by the motor101, and the optical head 102 irradiates the rotated optical disk 100with a laser beam. More specifically, the optical head 102 has an LD(laser diode), and the light beam emitted from the LD is focused on arecord mark on a track formed on the optical disk 100 via the objectivelens. Light reflected by the optical disk 100 is focused on aphotodetector via a condenser lens to be converted into a reproductionsignal. Note that the above record mark indicates both a mark recordedon a phase change recording layer with a change in reflectance and anembossed pit in recess/projection form.

The signal read by the optical head 102 is amplified by the reproductionamplifier 103. The amplified signal is sampled and converted intodigital data by an A/D converter 107. The clock signal used for samplingis generated by the PLL circuit 105. The PLL circuit 105 generates aclock signal from the PR-equalized reproduction signal.

The reproduction signal converted into the digital signal isPR-equalized by a PR equalizing section 108. The waveform equalizingsection 104 in FIG. 10 has a high frequency emphasis characteristic toamplify a densest signal with a small amplitude so as to facilitatebinary waveform slicing. In contrast to this, the PR equalizing section108 shown in FIG. 1 has a characteristic that forms an output waveforminto a waveform having undergone a predetermined specific interferencepattern.

A reproduction signal waveform and PR equalization will be describedwith reference to FIG. 2. Similar to “(a)” to “(d)” in FIG. 11, “(a)” to“(d)” in FIG. 2 respectively show recorded data, recorded data waveform,record marks, and a reproduction waveform. The waveform indicated by“(e)” in FIG. 2 is obtained by equalizing the reproduction waveformindicated by “(d)” in FIG. 2 by the equalizer on the basis of the PR(1,2, 2, 1) characteristic. The PR(1, 2, 2, 1) characteristic is acharacteristic in which impulse responses appear at four continuousidentification points at ratios of 1:2:2:1. Although not shown, thisapplies to other PR characteristics such as the PR (1, 2, 1)characteristic. In the PRML scheme, the signal deterioration caused bythe equalizer can be suppressed by performing waveform equalization tothe PR characteristic similar to the characteristic of the reproductionwaveform actually reproduced from an optical disk. For example, as shownin FIG. 3, the PR equalizing section 108 can be formed by a transversalfilter designed to multiply an output from each of cascaded delay units201 corresponding one sampling time by a predetermined coefficient in acorresponding one of multipliers 202 and adding the resultant outputsfrom the respective multipliers 202 by using an adder 203.

In a reproduction signal processing system based on the PRML scheme, aviterbi decoder 109, a typical maximum likelihood decoder, is generallyused as an identification unit to be placed after an equalizer. If areproduction signal is equalized to have the PR(1, 2, 2, 1)characteristic by the PR equalizing section 108, the viterbi decoderselects a series of values, from all series of values satisfying thePR(1, 2, 2, 1) characteristic, which exhibits the minimum error from aseries of sample values of an equalization waveform, and outputs binarydata (decoded data) corresponding to the selected series. FIG. 4 showsthis state. Referring to “(a)” in FIG. 4, the points indicated by thecircles indicate a series of sample values of an equalization waveform,and the solid line connecting them indicates a series of signal valuesselected by the viterbi decoder. The signal level of the selected seriesof signal values is indicated by “(b)” in FIG. 4. The decoded data basedon the PR(1, 2, 2, 1) characteristic is identified as indicated by “(c)”in FIG. 4.

In the PRML scheme, decoding is performed from a plurality of samplevalues instead of a single sample value, and the resultant data isidentified as a maximum likelihood series of signal values. This schemeis therefore resistant to signal deterioration components having nocorrelation between sample values.

In an optical disk apparatus, a resolution (MTF: Modulation TransferFunction) is restricted by the characteristics of an optical system. TheMTF has a characteristic that the amplitude of a reproduction signaldecreases as a record mark decreases in size. As the resolutionincreases, smaller record marks can be reproduced. On the other hand,the resolution is limited by the wavelength of an LD to be used or thenumerical aperture of an objective lens, the resolution cannot be freelyincreased. If, therefore, the recording density is increased under thecondition of restricted resolution, the amplitude of a reproductionsignal corresponding to a small record mark decreases.

FIG. 5 shows an example of the MTF characteristics. The ordinaterepresents the amplitude value of a reproduction signal; and theabscissa, the size of a record mark normalized with a size of one bit.If data is recorded by RLL(1, 7) modulation, a densest mark with thehighest density (with the smallest size) becomes two bits (2T), whereasa coarsest mark with the lowest density (with the largest size) becomeseight bits (8T). Therefore, a 2T reproduction signal with the highestdensity can acquire only a very small amplitude. With regard to anactual reproduction waveform, noise is added to this signal with thesmall amplitude and also influenced by interference from an adjacentsignal. In addition, since the densest signal is difficult to stablyrecord, the quality of the reproduced densest signal greatlydeteriorates in quality. For this reason, it is difficult to accuratelyobtain a correct value according to the conventional specifications of areproduction signal using the amplitude of a densest signal.

FIG. 6 shows an example of the reproduction signal waveform obtainedwhen random data having undergone RLL(1, 7) modulation is recorded on ahigh-density optical disk. The waveform shown in FIG. 6 can be obtainedby observing an output from the reproduction amplifier 103 with anoscilloscope. As is obvious from this observation result, an 8T signalwith the highest density can be measured as 18 in FIG. 6 from themaximum amplitude of the measurement result. On the other hand, theamplitude of a 2T signal with the highest density seems to be 12 in FIG.6 and is difficult to accurately measure because the signal amplitude issmall and influenced by interference. Therefore, this value isinadequate to evaluate a reproduction signal from a disk.

In the case of a high-density disk in which the amplitude of a signalwith the highest density becomes 20% or less (or 15% or less) of that ofa signal with the lowest density, attention is paid to the amplitude ofa 3T signal with the next highest density as compared with the 2T signal(i.e., a signal that reflects information recorded by forming a recordmark with a size larger than that of a record mark with the smallestsize by one rank). As the 3T signal amplitude indicated by “I3” in FIG.6, a sufficient amplitude can be ensured as compared with the case ofthe 2T signal. This value is therefore suitable for evaluation of ahigh-density optical disk.

A method of evaluating a reproduction signal from a high-density opticaldisk by using a quasi-densest signal with the second highest density anda coarsest signal with the lowest density will be described next withreference to FIG. 6.

Modulation amplitude is an evaluation item for a reproduction signal. Asthe resolution of the optical system of an optical disk apparatusdecreases, the MTF characteristic deteriorates. Although a decrease inresolution does not influence a signal with a low density much, thesignal amplitude of a signal with a high density greatly increases.Therefore, whether the resolution of the optical system satisfies aspecification or data is properly recorded on an optical disk by anapparatus having a predetermined resolution can be evaluated bycomparing the amplitude of a signal with a high density with that of asignal with a low density and obtaining a modulation amplitude.

In the case of a high-density optical disk on which data is recorded byRLL(1, 7) modulation, a 3T signal amplitude (I3) with the second highestdensity is defined as a quasi-densest signal by using an 8T signalamplitude (I8) which is a coarsest signal having the lowest densityaccording to the following equation. This makes it possible to stablyevaluate an optical disk and reproduction signal.

Asymmetry is another evaluation item for a reproduction signal. Toidentify a reproduction signal, the central level of the reproductionsignal must be constant regardless of the signal density. For thisreason, asymmetry is added as an evaluation item. If the identificationscheme used is changed from the PRML scheme because of an increase indensity, the signal level corresponding to each density becomesimportant in addition to the central level. Therefore, asymmetry isstill an important evaluation item. Asymmetry is influenced by theshapes of a mark and beam spot formed on an optical disk.

In the case of a high-density optical disk on which data is recorded byRLL(1, 7) modulation, asymmetry can be examined from the offset amountbetween the central level of a 3T signal which is a quasi-densest signalwith the second highest density and the central level of an 8T signalwhich is a coarsest signal with the lowest density. More specifically,asymmetry (SA) can be defined on the basis of the low-level side (L3L)and high-level side (L3H) of the 3T signal and the low-level side (L8L)and high-level side (L8H) of the coarsest signal amplitude according tothe following equation. This makes it possible to stably evaluate anoptical disk and reproduction signal.

As described above, in defining a modulation amplitude and asymmetryusing a densest signal, a decrease in densest signal level anddeterioration in quality make it difficult to perform stable evaluation.Using a quasi-densest signal with a density lower than a densest signalmakes it possible to perform stable evaluation. Although a signal withthe second highest density is suitable as a quasi-densest signal, asignal with another level may be used as long as the level differencebetween it and a densest signal is sufficient.

The above reproduction signal evaluation method will be summarized withreference to the flow chart of FIG. 8. As shown in FIG. 8, first of all,a reproduction signal is detected (ST11). More specifically, aninformation recording medium on which digital information is recorded byusing record marks with various sizes is played to detect a reproductionsignal reflecting the digital information. The magnitude of a firstreproduction signal, of the signals included contained in the detectedreproduction signal, which reflects digital information recorded by arecord mark with the largest size (i.e., a reproduction signal with thelowest recording density) is obtained (ST12). In addition, the amplitudeof a second reproduction signal, of the signals contained in thereproduction signal, which reflects digital information recorded by arecord mark with a predetermined size, other than the record mark withthe smallest size, (i.e., a reproduction signal with a predeterminedrecording density, other than the reproduction signal with the highestrecording density) is obtained (ST13). An evaluation value is thencalculated on the basis of the ratio of the amplitude of the firstreproduction signal to that of the second reproduction signal or thedifference between the central levels of the amplitudes (ST14). Thecharacteristic of the reproduction signal is evaluated with thisevaluation value (ST15).

In addition, by evaluating a reproduction signal using the abovereproduction signal evaluation method, an information recording mediumcan be provided, on which digital information is recorded such that areproduction signal that satisfies a predetermined evaluation conditioncan be obtained. Furthermore, an information reproduction apparatus canbe provided, which reproduces a reproduction signal satisfying apredetermined evaluation condition when playing such an informationrecording medium. For this reason, information can be properlyreproduced not only by a specific type of apparatus but also by varioustypes of reproduction apparatuses.

The evaluation method using an output signal from a reproductionamplifier before waveform equalization has been described so far.However, evaluation can also be made by using a signal after waveformequalization. An evaluation method using a signal after waveformequalization will be described next.

FIG. 4 shows examples of a series of signal values of a reproductionsignal waveform and a series of sample values of an equalizationwaveform. FIG. 7 shows an example of the histogram obtained byclassifying and collecting sample values after equalization in the leveldirection. In the case of the PR(1, 2, 2, 1) characteristic, sevendistribution peaks appear. Let P0 to P6 be the values of the respectivelevel distribution peaks. If a modulation amplitude and asymmetry areadequate and waveform equalization is properly performed, the values P0to P6 become equal to ideal values C0 to C6 calculated from the PR(1, 2,2, 1) characteristic. The ideal values have the following relationshipbecause all the intervals between the adjacent level distribution peaksare equal:

If, however, an optical disk is not properly formed or an apparatus isnot adequately adjusted, a modulation amplitude or asymmetry deviationoccurs. As a consequence, the values of these level distribution peaksdeviate from the ideal values. If the sample values deviate from theideal values, an error occurs in the viterbi decoder in the process ofcalculating an optimal data series. An increase in errors for the peaksgenerates an identification error. Therefore, the deviations of therespective peak values from the corresponding ideal values are obtained.This makes it possible to evaluate the optical disk and reproductionsignal.

Methods of obtaining evaluation values from the level distribution peaksobtained with respect to the series of sample values of the equalizationwaveform in FIG. 7 will be described next.

According to the first method, the differences between the respectivelevel distribution peak values and the corresponding ideal values areobtained, and a characteristic is evaluated in accordance with the sumtotal of the absolute values of the differences. In this case, thesignal characteristic (S1) is given by ##EQU1##

As described above, the characteristic may be evaluated by using theaverage value of the absolute values as well as the sum total of theabsolute values.

According to the second method, the differences between the respectivelevel distribution peak values and the corresponding ideal values areobtained, and a characteristic is evaluated on the basis of the sumtotal of the squares of the differences. In this case, the signalcharacteristic (S2) is given by ##EQU2##

As described above, the characteristic may be evaluated by using theaverage of the squares as well as the sum total of the squares.

The above evaluation method using an equalization waveform is effectiveas a relatively easy evaluation method incorporating equalizerperformance. To reflect an error rate after identification in ameasurement result with more likelihood, the above processing ispreferably performed after assigning weights to the differencesassociated with the respective level distribution peaks in considerationof the properties of the signal and the identification method used. Inthis case, equations (6) and (7) can be modified into equations (8) and(9), letting W0 to W6 be the weighting factors assigned to therespective level distribution peaks. ##EQU3##

Examples of parameters associated with a weight factor determiningmethod will be described next.

Consider the distributions of the respective peaks appearing in thehistogram. The variances of the respective distributions are notnecessarily equal to each other owing to the influences of factors thatcause noise components and the frequency characteristics of waveformequalization. Even with the same deviation amounts of the respectivepeak values from the corresponding ideal values, the sample value at anend of a distribution with a large variance greatly differs from theideal value. As a consequence, this distribution greatly overlaps anadjacent distribution, resulting in an increase in the possibility of anerror in the identification step. Therefore, a large weighting factor isassigned to a peak with a large variance, and a small weighting factoris assigned to a peak with a small variance, thereby increasing thedegree of correlation between the evaluation result and the error rate.

In addition, according to the modulation characteristics of recordeddata, the occurrence frequencies of the respective level distributionsare not necessary equal to each other. It is regarded that as the peaksof level distributions with a high occurrence frequency deviate, theoverall error rate is influenced more. In general, recorded data isoften converted into random data to eliminate a signal offset. If,therefore, a modulation scheme is determined, the actual occurrencefrequency distribution can be regarded to be almost equal to theoccurrence frequency distribution obtained when random data ismodulated. By using the ratios of the occurrence frequencies of therespective levels with respect to random data as weighting factors, thedegree of correlation between the evaluation result and the error ratecan be increased.

According to the characteristics of viterbi decoding on the rear stage,the degrees of influences of deviations on errors after identificationare not necessarily equal to each other depending on the signal levels.For example, in the case of the PR(1, 2, 2, 1) characteristic, even ifthe minimum level P0 of the equalization waveform becomes smaller thanthe ideal value C0 or the maximum level P6 become larger than the idealvalue C6, no error easily occurs in viterbi decoding. In contrast tothis, a deviation at an intermediate level such as P1, P2, P4, or P5relatively tends to become a factor that causes an error. The degree ofinfluence on an error after identification with consideration given tothe characteristics of viterbi decoding is obtained by measuring thelevel of an equalization waveform when an error actually occurs. A largeweighting factor is set for a level at which the degree of influence onan error is large, and vice versa. This makes it possible to increasethe degree of correlation between the evaluation result and the errorrate.

Each method described above is based on addition of all peaks inaccordance with the number of peaks defined by the PR characteristic.Alternatively, to simplify the above processing, addition may beperformed for only some peaks, and the resultant value may be used as atypical evaluation value. Obviously, comprehensive weighting factors maybe finally determined by combining the degrees of influence on theseparameters.

Addition is performed after level distribution peaks are obtained fromthe histogram obtained by classifying and collecting sample values of anequalization waveform in the level direction and assigning weights tothe differences between the respective peak values and the correspondingideal values. With this operation, as compared with the case whereevaluation is performed with an error rate after identification,evaluation can be performed more easily with a proper correlation withthe error rate including equalizer performance.

The above reproduction signal evaluation method will be summarized withreference to the flow chart of FIG. 9. As shown in FIG. 9, first of all,a reproduction signal is detected (ST21). More specifically, aninformation recording medium on which digital information is recorded byusing record marks with various sizes is played by an optical means, anda reproduction signal reflecting the digital information is detected.The detected reproduction signal is equalized on the basis of apredetermined partial response characteristic to generate a multilevelequalization signal (ST22). The amplitude level distribution of theequalization signal is then obtained, and a plurality of actuallymeasured peak values are calculated from this level distribution (ST23).A plurality of ideal peak values are calculated from the predeterminedpartial response characteristic (ST24). The differences between therespective actually measured peak values and the corresponding idealvalues are obtained (ST25). An evaluation value is calculated on thebasis of the sum total or average of the absolute values of thedifferences between the respective peak values or the sum total oraverage of the squares of the differences (ST26). The characteristic ofthe reproduction signal is evaluated by this evaluation value (ST27).

In addition, by evaluating a reproduction signal using the abovereproduction signal evaluation method, an information recording mediumcan be provided, on which digital information is recorded such that areproduction signal that satisfies a predetermined evaluation conditioncan be obtained. Furthermore, an information reproduction apparatus canbe provided, which reproduces a reproduction signal satisfying apredetermined evaluation condition when playing such an informationrecording medium. For this reason, information can be properlyreproduced not only by a specific type of apparatus but also by varioustypes of reproduction apparatuses.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A reproduction signal evaluation method comprising: a first step ofplaying an information recording medium on which digital information isrecorded with record marks having various sizes by using optical means,and obtaining a reproduction signal reflecting the digital information;a second step of obtaining a first reproduction signal, of signalscontained in the reproduction signal, which reflects digital informationrecorded with a record mark having a largest size, on condition that thedigital information is recorded by RLL (1, n) modulation; a third stepof obtaining a second reproduction signal, of signals contained in thereproduction signal, which reflects digital information recorded with arecord mark having a predetermined size other than a record mark havinga smallest size, on condition that the digital information is recordedby the RLL (1, n) modulation; a fourth step of obtaining an evaluationvalue on the basis of the first and second reproduction signals; and afifth step of evaluating a characteristic of the reproduction signal onthe basis of the evaluation value.
 2. An information recording medium onwhich digital information is recorded such that a reproduction signalsatisfying a predetermined evaluation condition is obtained when thereproduction signal is evaluated by a reproduction signal evaluationmethod comprising: a first step of playing an information recordingmedium on which digital information is recorded with record marks havingvarious sizes by using optical means, and obtaining a reproductionsignal reflecting the digital information; a second step of obtaining afirst reproduction signal, of signals contained in the reproductionsignal, which reflects digital information recorded with a record markhaving a largest size, on condition that the digital information isrecorded by RLL (1, n) modulation; a third step of obtaining a secondreproduction signal, of signals contained in the reproduction signal,which reflects digital information recorded with a record mark having apredetermined size other than a record mark having a smallest size, oncondition that the digital information is recorded by the RLL (1, n)modulation; a fourth step of obtaining an evaluation value on the basisof the first and second reproduction signals; and a fifth step ofevaluating a characteristic of the reproduction signal on the basis ofthe evaluation value.
 3. An information reproduction apparatus forreproducing a reproduction signal satisfying a predetermined evaluationreference from an information recording medium on which digitalinformation is recorded such that a reproduction signal satisfying apredetermined evaluation condition is obtained when the reproductionsignal is evaluated by a reproduction signal evaluation methodcomprising: a first step of playing an information recording medium onwhich digital information is recorded with record marks having varioussizes by using optical means, and obtaining a reproduction signalreflecting the digital information; a second step of obtaining a firstreproduction signal, of signals contained in the reproduction signal,which reflects digital information recorded with a record mark having alargest size, on condition that the digital information is recorded byRLL (1, n) modulation; a third step of obtaining a second reproductionsignal, of signals contained in the reproduction signal, which reflectsdigital information recorded with a record mark having a predeterminedsize other than a record mark having a smallest size, on condition thatthe digital information is recorded by the RLL (1, n) modulation; afourth step of obtaining an evaluation value on the basis of the firstand second reproduction signals; and a fifth step of evaluating acharacteristic of the reproduction signal on the basis of the evaluationvalue.